d or unsubstituted alkylene group containing 1 to 6 carbon atoms, R5 and R6 are identical or different and denote hydrogen atoms, alkyl radicals containing 1 to 6 carbon atoms, alkoxy radicals containing 1 to 4 carbon atoms, or hydroxyalkyl radicals containing 1 to 3 carbon atoms or hydroxyl groups, R7 is a hydrogen atom or an alkyl radical containing 1 to 6 carbon atoms, R8 is a hydrogen atom, an